EFFECTS OF INOCULATION METHOD AND SIZE OF RHIZOBIUM-MELILOTI POPULATION IN THE SOIL ON NODULATION OF ALFALFA

Field experiments were conducted with alfalfa (Medicago sativa L. 'Peace') planted on a Black Solod (Landry CL) with pH 5.8 in 3 consecutive years. The same experiment was conducted at two adjacent sites in each year. The population of Rhizobium meliloti in the soil was modified at one site by applying and incorporating a suspension of an indigenous isolate to increase the size to 10(5) g-1. The other site was on unamended soil with a normal indigenous population of less than 10(2) g-1. Granular inoculant was applied with the seed or below the seed, or, granular or liquid inoculant was applied beside the row after stand establishment. The soil-placed inoculant treatments were compared with an uninoculated control and a standard seed-applied inoculation treatment. All inoculants were prepared with Rhizobium meliloti NRG-61 which had been selected for low-pH tolerance. Root weight, shoot weight, nodule weight, nodule numbers and strain occupancy of nodules were measured in September of the establishment year and again the following June. Granular inoculant applied with or below the seed resulted in greater nodule weights, nodule numbers and percent nodule occupancy at both sites. Granular inoculant applied with or below the seed was more effective at the site with the normal indigenous population of R. meliloti than at the site with the modified population. When granular inoculants were applied, nodule weights were lower on the site with the modified indigenous population than on the soil with the normal indigenous population. Granular inoculants resulted in significant yield increases over the standard seed-applied inoculant only at the site with the normal indigenous population. There were no substantial differences between the sites in the proportion of nodules occupied by the inoculant strain. These results show that strain occupancy measurements do not necessarily reflect the beneficial effect of inoculation.